The portable communications devices of modern telecommunications systems need antennas that should fulfil a number of requirements, some of which appear to be mutually contradictory. The antenna should be small, light and easy to manufacture in large-scale mass production at low cost. The antenna should have resonant frequencies in multiple frequency ranges, which in cellular communications systems are up to 1000 MHz apart from each other, and in FM radio reception can be as low as below 100 MHz. The input impedance of the antenna should match the impedance of an antenna port of a transceiver or receiver over a relatively wide frequency band. Losses in the antenna, caused by conduction losses in the conductive parts of the antenna and dielectric losses in the supporting and surrounding materials, should be as low as possible.
Especially the requirement for a small size causes difficulties. In general, the smaller the antenna is made, the narrower its impedance bandwidth becomes. The miniaturization requirements concern not only the radiating antenna part; also the ground plane related to the antenna structure should be as small as possible.
Interesting developments in this field have been introduced in the form of fractal antennas. A fractal is a self-similar structure, which means that a small part of the structure is a scaled-down copy of the original structure. A fractal antenna is one where a radiating antenna element has the shape of a fractal curve. The self-similarity of the structure often leads to multifrequency operation, because at a higher frequency and thus a smaller wavelength a smaller part of the antenna replicates the resonant characteristics of the whole antenna at a lower frequency. A fractal curve is also relatively long compared to the overall two-dimensional area it occupies. This is advantageous, because the end-to-end length of a line-shaped antenna radiator must be at least one quarter of the wavelength at the desired resonant frequency. It is relatively easy to make a small-sized antenna structure by using a tightly meandering fractal curve as the radiating part.
Known prior art patents and patent applications involving fractal antenna design include U.S. 20020190904 A1; U.S. Pat. No. 6,476,766; U.S. Pat. No. 6,452,553; U.S. Pat. No. 6,445,352; U.S. Pat. No. 6,140,975; U.S. Pat. No. 6,127,977; U.S. Pat. No. 6,104,349; WO 2004/001894; WO 03/023900; WO 01/54225; WO 01/54221; WO 99/57784; WO 97/06578; EP 1 313 166; EP 1 258 054; EP 1 227 545; EP 1 223 637 and ES 2 112 163. A list of known scientific publications is provided below at the end of the detailed description. Some of these publicly available documents also introduce the concept of space-filling curves. A space-filling curve is not a fractal, because it does not replicate itself in smaller scale. However, much like many fractals, space filling curves are defined by recursive replacement rules. There is a certain degree of similarity between the recursive iterations when a space-filling curve is developed. By proceeding through a large number of iterative replacement rounds it is mathematically possible to make a space-filling curve fill in a given space up to any given arbitrary percentage. A mathematically more accurate description of a genuine space-filling curve is a function that continuously maps the unit interval onto a bounded region of higher dimension.
The problems of known fractal and space-filling antennas are usually related to modest efficiency and too narrow bandwidth. Efficiency problems can be tracked to the requirement of making the meandering conductive trace in the antenna relatively long, in order to achieve an impedance match to the antenna port of a transceiver or receiver at required operating frequencies.